The invention concerns a process for producing an elongated single- or multi-core superconductor having at least one conductor core embedded in an Ag matrix, which conductor core has a bismuth-containing superconductor material with a 2212 or 2223 type high-T.sub.c phase. The process comprises the following steps:
A structure is prepared from the matrix material and at least one core, made of the precursor of the superconductor material, PA1 the structure is converted to a raw conductor using a special cross-section-reducing treatment, and PA1 the raw conductor is subjected to an annealing process with controlled melting in an oxygen-containing atmosphere to form the high-T.sub.c phase.
The invention also concerns a superconductor manufactured by this process.
A similar process and a superconductor produced by this process are known from "Supercond. Sci. Technol.," Vol. 5, 1992, pp. 591-598.
Known superconducting metal oxide compounds with high transition temperatures T.sub.c of over 77 K, which are therefore also referred to as high-T.sub.c superconductor materials (for short: HTSL materials), include in particular cuprate based on bismuth material Bi--Sr--Ca--Cu--O (BSCCO) or B(Pb)--Sr--Ca--Cu--O (B(P)SCCO) system. In this system, two superconducting phases appear, which differ by the number of copper-oxygen lattice planes (layers) of the crystal unit cell. A superconducting phase with the approximate composition Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+y, (referred to as a 2-layer/85-K or 2212-phase) has a transition temperature T.sub.c of approximately 85 K, while the transition temperature of a superconducting phase with the approximate composition Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10+x, (referred to as a 3-layer or 100-K or 2223 phase) is approximately 110 K.
Attempts have been made to manufacture elongated superconductors in wire or tape form with these HTSL materials. A process considered suitable for this purpose is referred to as "powder-in-tube" technology, known in principle from the manufacture of superconductors with the traditional superconducting material Nb.sub.3 Sn (see, for example, German Auslegeschrift 1 257 436). According to this method, a powder made from a precursor of the HTSL material, which in general contains little or none of the desired superconducting high-T.sub.c phase, is filled into a tubular carrier or into a matrix made of normally conducting material, in particular of Ag or an Ag alloy. The structure thus obtained is then brought to its final dimensions by means of forming processes, which may be interrupted by at least one heat treatment. Then the wire- or tape-shaped raw conductor is subjected to at least one annealing operation, performed at least partially in an oxygen-containing atmosphere, e.g., air, to adjust or optimize its superconducting characteristics or to form the desired high-T.sub.c phase (see, for example, "Supercond. Sci. Technol.," Vol. 4, 1991, pp. 165-171).
If a plurality of such tape- or wire-shaped high-T.sub.c superconductors or their conductor precursors are bundled together, conductors with a plurality of superconducting cores, referred to as multi-core or multi-filament conductors, can be obtained, which offer a series of advantages for technical applications (see the article by M. Wilhelm et al. entitled "Fabrication and Properties of Multifilamentary BiPbSrCaCuO-2223 Tapes" of the "International Symposium on Superconductivity" (ISS'93), Hiroshima, Japan, Oct. 26-29, 1993).
It is known from the aforementioned reference from "Supercond. Sci. Technol.," Vol. 5 that the textural properties of such reaction-annealed tape conductor cores can be improved, thereby increasing the critical currents, and the dependence of the critical current densities on the magnetic field can be reduced by partial melting and subsequent controlled crystallization of the ceramic. The corresponding process is known as the PFDR (phase formation-decomposition-recovery) process. According to this process, a nitrate mixture of suitable composition is calcined at 830.degree. C. and, after reaction annealing at 845.degree. C. in air, it yields basically the 2-layer compound and alkaline earth cuprate. With this reaction product, a tape-shaped raw conductor is prepared with a silver shell according to the powder-in-tube process and annealed at 835.degree. to 838.degree. C. to form the 3-layer phase. A one-time short melting of the conductor core at 860.degree. C. between the reaction annealing and a first secondary annealing results in a higher core density and a fine distribution of 2-layer residues and intergranular minority phases in a 3-layer matrix. Thus "pinning centers" are obtained and increased magnetic field-independence of the superconductance at 77 K is ensured. Additional compacting and annealing at 838.degree. C. is performed after the first secondary annealing at 838.degree. C.
A similar process with a PFDR process is also disclosed in WO 93/22799. In this known process, the 2223 phase obtained in the conductor structure is also briefly melted at approximately 860.degree. C. and post-annealed at approximately 839.degree. C. to thus obtain a small proportion of the 2212 phase dispersed in the 2223 phase, which serves as a matrix.
Superconductor manufacturing on an industrial scale according to the PFDR process presents problems, however, because the adjustment of the optimum degree of partial melting is extremely critical. The proper volume of melted material is, however, decisive for obtaining a conductor with high core density and a high current capacity. For a given composition of the core ceramic, the melted volume depends on the temperature and the annealing time, but also on the conductor dimensions. The greater the core volume, the lower the melting temperature must be. Low-viscosity melt, which is required for quick and homogeneous material distribution and thus pore-and contaminant-free growth, is produced in sufficient amounts only in a temperature range where the superconducting phases in the B(P)SCCO superconductor decompose again. If a relatively high melting temperature (860.degree. C.) is provided, as in the case of the known PFDR process, there is the danger that the decomposition of the desired superconducting phase can no longer be reversed by subsequent annealing at a relatively lower temperature level.